Coupling constants electronegativity

Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3,4 and V3.5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The V values correlate quite well with rr-bond orders calculated by MO methods (74CJC833). 

Table 2.6. Structural features (carbon hybridisation, electronegativity, ring size) and typical one-bond CH coupling constants Jch (Hz) ...

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

Haasnoot, C. A. G., De Leeuw, F. A. A. M., De Leeuw, H. P. M., Altona, C. Relationship between proton-proton NMR coupling constants and substituent electronegativities. III. Conformational analysis of proline rings in solution using a generalized Karplus equation. Biopolymers 1981, 20,1211-1245. 

As indicated, the magnitudes of three-bond F—F and F—H coupling constants are observed to vary as a function of the sum of the electronegativities of the other substituents that are on the two carbons in question, with the absolute values of these coupling constants... 

TABLE 2.5. Vicinal Coupling Constants as a Function of Multiple Electronegative Substituents... 

A chlorine substituent at the 2-position of a 1,1-difluoroalkene will deshield the fluorines modestly, but as seen in Scheme 4.46, a vicinal alkoxy group shields both fluorines, the trans fluorine more greatly. With the additional electronegative substituent, three-bond F—H coupling constants for such compounds become even smaller. 

The coupling constants increase with increasing sum of the electronegativities (E EN) of the substituents. A least square fit results in the nonlinear correlation (Pauling electronegativities)... 

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